The present invention relates to an electrolyte for the galvanic deposition of stress-relieved/low-stress, crack-free ruthenium layers.
In a further aspect, the present invention relates to the method of producing an electrolyte for the galvanic deposition of stress-relieved, crack resistant ruthenium layers and the use of pyridine and N-alkylated pyridinium salts for that purpose.
Galvanic ruthenium baths are based as a rule on ruthenium(III) compounds in aqueous-acidic solution. Ruthenium(III) chloride serves primarily as the ruthenium source. For a good many years ruthenium baths have been used which contain amidosulfuric acid (sulfamic acid) and/or ammonium sulfate and in which the ruthenium is present in complexed form. The ruthenium can be present therein as a complex with amidosulfuric acid, with an organic acid or in the form of the complex [Ru.sub.2 NX.sub.8 (H.sub.2 O)].sup.3-, wherein X.dbd.halogen, especially the ruthenium nitridochloro complex ("RuNC salt"). Such baths have proved that they are stable and unobjectionable and yield ruthenium coatings with good adhesion. More detailed information about the pertinent state of the art can be obtained e.g. from publications such as Galvanotechnik 81 (8), 2742-2744 (1990), Chem. Ing. Tech. 50, 296-298 (1978) as well as the patent documents DE 22 61 944 (corresponding to U.S. Pat. No. 3,793,162), DE 20 00 410 (corresponding to Great Britain 1191435), and DE 18 03 524 corresponding to U.S. Pat. No. 3,576,724, all of which are relied on and incorporated herein by reference.
Inner stresses are built up in the galvanic deposition of ruthenium layers, which increasingly occurs with increasing thickness of the deposited coating. This can lead to the formation of cracks. Therefore, dense, crack-free, well-adhering, shiny ruthenium coatings can be produced only to a thickness of approximately 2 to 3 .mu.m with current galvanic baths and customary pretreatment techniques and deposition techniques. Inner stresses of up to 600 N/mm.sup.2 can readily occur in layer thicknesses up to this order of magnitude. Previously known measures for reducing the inner stresses, e.g. by modifying the ruthenium complex, changing the electrolyte compositions, adding stress-reducing additives, varying the deposition conditions, etc. always entailed other disadvantages such as in particular a drastic lowering of the current flow.
An object of the invention therefore is to find additives which act in a stress-reducing manner in electrolytes for the galvanic deposition of ruthenium layers so that crack-free layers can be obtained, even in greater thickness. These additives, however, should not exert a negative impact on the electrolyte or on the deposition itself.